A device which varies the distribution ratio of a drive torque between the main drive wheels and extra drive wheels of a four-wheel drive vehicle is disclosed for example in Tokkai Hei 2-270641 published in 1990 by Japanese Patent Office.
In this device, the main drive wheels are directly driven, while the extra drive wheels and engine drive shaft are connected via a wet type multi-plate friction clutch.
The torque distribution ratio to the extra drive wheels is varied by varying the grip force of the clutch according to a control signal. The grip force of the clutch is increased the larger the difference between the rotation speeds of the front and rear wheels, i.e. the larger the slip of the rear wheels which are the main drive wheels. As a result, the distribution ratio of the drive force to the front wheels, which are the extra drive wheels, is increased, and the slip of the rear wheels is promptly terminated.
In such a four-wheel drive vehicle, when the vehicle makes a turn during four-wheel drive, the difference in the rotation radius of the front and rear wheels causes a torsion in the suspension, tires and vehicle body.
If the driver stops the vehicle in the turn and turns off the engine, the grip force of the clutch falls to zero because the clutch grip force comes from an oil pump driven by the engine, and as the front wheels suddenly become free, the torsion in the suspension, tyres and vehicle body is immediately released. The release of this torsion causes the vehicle body to vibrate and oscillate in a top/bottom and front/back direction, and this gives an uncomfortable feeling to the driver and passengers of the vehicle.